Today's chemical world rely mainly on fossils fuel resources. However, due to depleting fossils fuel resources and ever increasing emission of greenhouse gases there is a strong motivation to substitute fossils fuel resources with renewable resources. Currently, in the era of petroleum economy, there exists an intense focus on the production of transportation fuels and other platform chemicals from biomass. This focus can be ascribed to a desire to significantly lower the emission of greenhouse gases thereby minimizing global warming and also to renounce dependence on fossils fuels.
Plant oil derived fatty acids can be a suitable alternative as they have characteristic long methylene sequence with an internal double bond. This internal double bond provides an excellent opportunity to further functionalize the plant oil to useful chemicals and materials. Therefore it has been a long cherished dream of organometallic chemists to isomerize the double bond to terminal position and functionalize it selectively. However isomerization of internal double bond to terminal olefins is thermodynamically unfavorable which makes the terminal functionalization of plant oil a challenging transformation. A very few attempts have been made to address this bottle-neck in the past. Isomerizing hydroformylation of fatty acid methyl esters (FAMES) was first reported by Behr et al. and very limited success was achieved (26% terminal aldehyde). The conversion is slightly improved in case of isomerizing hydroboration (45%). Isomerizing metathesis is of significant importance and is successfully up scaled and various products such as candle waxes, cosmetics etc. are claimed to be produced by this transformation. The most successful example is isomerizing alkoxycarbonylation (>95% terminal selectivity) which was first introduced by Cole-Hamilton and later developed by Mecking et al. The mechanism of this particular process has been investigated by Mecking and coworkers.
India is the second largest producer of Cashew Nut Shell Liquid (CNSL) with an annual production of more than 20,000 tones. CNSL is a versatile byproduct of cashew industry and is a renewable and inexpensive resource. It is obtained from spongy mesocarp of cashew nut shell. The CNSL can be obtained by extraction in hot oil; liquid extraction (solvents); mechanical expulsion from the shells or by vacuum distillation. The variability of composition depends of extraction method but in general, the composition of natural CNSL is a mixture of anacardic acid, cardanol, cardol and 2-methyl-cardol in smaller quantities. Decarboxylation of anacardic acid at 140° C. followed by vacuum distillation results in cardanol in pure form. The relative composition of monoene, diene and triene in cardanol is established with HPLC. Diene and triene can selectively be hydrogenated to monoene when cardanol is subjected to selective hydrogenation in presence of RuCl3 and isopropanol.
Article titled “Tandem hydroformylation/hydrogenation of alkenes to normal alcohols using Rh/Ru dual catalyst or Ru single component catalyst” by Kohei Takahashi et al. published in Journal of the American Chemical Society, 2012, 134, pp 18746-18757 reports detailed investigation about Ru-catalyzed hydrogenation of undecanal under H2/CO pressure clarified different kinetics from the hydrogenation under H2 and gave a clue to design more active hydrogenation catalysts under H2/CO atmosphere. The solely Ru-catalyzed normal selective hydroformylation/hydrogenation is also reported.
Article titled “Tandem isomerization/hydroformylation/hydrogenation of internal alkenes to N-alcohols using Rh/Ru dual- or ternary-catalyst systems” by Yamato Yuki et al. published in Journal of the American Chemical Society, 2013, 135, 17393-17400 reports A one-pot three-step reaction, isomerization/hydroformylation/hydrogenation of internal alkenes to alcohols, was accomplished by employing a Rh/Ru dual-catalyst system. By using a combination of Rh(acac)(CO)2/bisphosphite and Shvo's catalyst, (Z)-2-tridecene was converted to 1-tetradecanol in 83% yield with high normal/iso selectivity (n/i=12). The method was applicable to other internal alkenes, including functionalized alkenes, such as an alkenol and an alkenoate.
Article titled “Isomerizing hydroformylation of fatty acid esters: Formation of ω-aldehydes” by Arno Behr et al. published in European Journal of Lipid Science and Technology, 107 (2005) 213-219 reports The isomerizing hydroformylation of fatty acid esters to oleochemicals with an additional o-standing aldehyde group can be performed at a relatively low temperature and a synthesis gas pressure of 20 bar. In the case of oleic acid ester, the best yield of linear aldehyde is 26%; in the case of linoleic acid ester, it is 34%. For both fatty compounds, a strong hydrogenation side reaction is observed, which can be explained by a steering effect of the ester group. The ester function of the fatty compounds makes hydroformylation in the surrounding area of this group impossible. Reactions with the model substances ethyl crotonate and ethyl sorbate showed that hydrogenation predominates, leading to the corresponding saturated compounds.
Article titled “Selective ethenolysis and oestrogenicity of compounds from cashew nut shell liquid” by Jennifer Julis et al. published in Green Chem., 2014, 16,2846 reports the ethenolysis of cardanol (2), a waste product from cashew kernel production, was carried out using a variety of metathesis catalysts. Surprisingly, the best activities and selectivities could be observed with ruthenium based 1st generation type catalysts converting cardanol (2) almost completely to the corresponding 1-octene (6) and 3-non-8-enylphenol (4), a potential detergent precursor. Detailed investigation of the reaction system showed that the high activity and selectivity were due to a combination of ethenolysis and internal self-metathesis of the unsaturated cardanol mixture, 2. Self-metathesis of cardanol (2) containing three double bonds led to the formation of 3-non-8-enylphenol (4) and 1,4-cyclohexadiene (7). The latter was crucial for a high selectivity and activity in the ethenolysis, not only of cardanol (2), but also of other substrates like methyl oleate (10) when using ruthenium based 1st generation catalysts. The endocrine disrupting properties of 3-nonylphenol and related compounds are compared.
Article titled “Isomerization-Hydroformylation Tandem Reactions” by Marcelo Vilches-Herrera et al. published in ACS Catalysis, 2014, 4, 1706-1724 reports Rhodium-Catalyzed isomerization of aldehydes. Conversion and product distribution in the Hydroformylation of isomeric C8-olefins with an unmodified Cobalt catalyst.
Developing similar terminal selectivity in a one pot isomerizing hydroformylation reaction will be of utmost significance, as it would provide a powerful tool for hetero-functionalization of plant oils along with complete feedstock utilization. Therefore, there is need to develop isomerizing hydroformylation of plant oils using metal precursor.